1. Field of the Invention
This invention concerns a ferroelectric liquid crystal display device, and in particular a liquid crystal display device wherein unevenness of the display color due to non-uniformity of cell thickness, is eliminated.
2. Description of the Related Art
A ferroelectric liquid crystal display device which operates in a new birefringence display mode using a combination of a ferroelectric liquid crystal and polarizers is disclosed in Japanese Unexamined Patent Publication No. 56-107216. This device has the following characteristics:
(1) High speed response (of the order of several tens microseconds), PA0 (2) Memory capability for alignment.
In particular, the memory capability (2) makes possible a high capacity display of over 400 lines without any cross-talk.
We shall describe the principle of ferroelectric liquid crystal display based on the devices of the prior art.
As shown in FIG. 1, a 1st and 2nd polarizer 3 and 4 are arranged respectively on either side of a cell 5 consisting of a liquid crystal composition enclosed between a pair of substrates. When an electric field is applied to this cell 5 in the direction marked 600, the director 500 points in the direction marked 501. When an electric field is applied in the direction marked 601, on the other hand, director 500 points in the direction marked 502. In FIG. 1, the 1st polarizer 3 is arranged such that its polarizing axis 31 is aligned with direction 501, while the 2nd polarizer 4 is aligned such that its polarizing axis 41 is perpendicular to axis 31 of 1st polarizer 3.
We shall now explain the polarization of the light which passes through this liquid crystal display device. First, natural light passes through 1st polarizer 3 and thereby becomes linearly polarized light 103. This light 103 then is incident on the liquid crystal cell 5. When an electric field is applied to cell 5 in the direction marked 600, the director 500 is aligned in the direction 501, and therefore the light 103 passes through cell 5 without any change of polarization. The light 105 which has left cell 5 is therefore linearly polarized. The plane of vibration of this linearly polarized light 105 is mutually perpendicular to polarizing axis 41 of 2nd polarizer 4. As a result, the light 105 is unable to pas through polarizer 4, and the liquid crystal display is then dark. On the other hand, when an electric field is applied to cell 5 in the direction marked 601, the director 500 points in direction 502 The linearly polarized light 103 incident on the cell 5 is thereby generally changed into elliptically polarized light. Thus the light 105 which leaves cell 5 is elliptically polarized light. The result is that part of the light 105 passes through the 2nd polarizer 4, and the liquid crystal display thus appears bright.
In the ferroelectric liquid crystal display, therefore, the orientation of the director is selected by the polarity of the external electric field, and effectively switches the light on and off. This type of method is known as birefringence mode display. In the meantime, we set forth the case where polarizing axis 31 of polarizer 3 is parallel to the orientation of the director of the liquid crystal when a voltage is applied to a liquid crystal cell. However, it is well known that it is not always necessary to arrange the polarizing axis parallel to the orientation of the director.
Now the transmission of the light when the birefringence mode display having the optical arrangement as shown in FIG. 1 appears bright, depends on the product of optical anisotropy of the liquid crystal, .DELTA.n, and the thickness of the liquid crystal layer d (this is known as the retardation value). The light transmission T (.lambda.) of the device in the bright condition under the cross-Nicol, is given by the following equation: EQU T (.lambda.)=T.sub.0 .multidot.sin.sup.2 (.pi..DELTA.nd/.lambda.)
where .lambda. is the wavelength of the light, and T.sub.0 is a constant.
We calculated the dependence of transmission of the liquid crystal cell on cell thickness for the case .DELTA.n=0.14. FIG. 2 shows the results. In FIG. 2, in the region of a cell thickness of 2 .mu.m, the transmissivities of red, blue and green light in the transmitted light are nearly 100%, and a white display is therefore obtained. In the region of a cell thickness of 2.2 .mu.m, however, only a light having wavelength nearer yellow has a higher transmissivity, and the color of the display therefore changes to light yellow. Further, when the cell thickness is in the region of 2.3 .mu.m, the color of the display becomes light purple.
The color of the display in the bright condition therefore changes considerably for a mere 0.1 .mu.m variation in cell thickness. In practice, it is recognized that if there is a +0.05 .mu.m difference in the thickness of a manufactured ferroelectric liquid crystal cell, it will be perceived as an unevenness in the color of the display, and this unevenness is undesirable. To suppress it and render the color uniform, it is thus necessary to control manufacture such that the thickness of the liquid crystal layer (cell thickness) is within a tolerance of +0.05 .mu.m. Meanwhile, the cell thickness of a ferroelectric liquid crystal device is normally approx. 1-5 .mu.m. This is very thin in comparison to the cell thickness of 5-8 .mu.m of the supertwisted liquid crystal display devices which are currently being mass produced, and it is consequently very difficult to control the thickness of the layer such that it is within a tolerance of +0.05 .mu.m over the whole surface of the liquid crystal panel.